A mass spectrometry (MS) system in general includes an ion source for ionizing components of a sample of interest, a mass analyzer for separating the ions based on their differing mass-to-charge ratios (or m/z ratios, or more simply “masses”), an ion detector for counting the separated ions, and electronics for processing output signals from the ion detector as needed to produce a user-interpretable mass spectrum. Typically, the mass spectrum is a series of peaks indicative of the relative abundances of detected ions as a function of their m/z ratios. The mass spectrum may be utilized to deter the molecular structures of components of the sample, thereby enabling the sample to be qualitatively and quantitatively characterized.
One example of an ion source is an electron ionization (EI) source. In a typical EI source, sample material is introduced into a chamber in the form of a molecular vapor. A heated filament is employed to emit energetic electrons, which are collimated and accelerated as a beam into the chamber under the influence of a potential difference impressed between the filament and an anode. The sample material is introduced into the chamber along a path that intersects the path of the electron beam. Ionization of the sample material occurs as a result of the electron beam bombarding the sample material in the region where the sample and electron paths intersect. The primary reaction of the ionization process may be described by the following relation: M+e−→M*++2e−, where M designates an analyte molecule, e− designates an electron, and M*+ designates the resulting molecular ion. That is, electrons approach a molecule closely enough to cause the molecule to lose an electron by electrostatic repulsion and, consequently, a singly-charged positive ion is formed. A potential difference is employed to attract the ions formed in the chamber toward an exit aperture, after which the resulting ion beam is accelerated into a downstream device such the mass analyzer or first to an intervening component such as an ion guide, mass filter, etc.
In the widely used cross-beam, or Nier-type, EI source, the ion beam is generated in a direction orthogonal the electron beam. This type of design is prone to loss of ions, due to a large number of ions being drawn out to the filaments or defocused and neutralized (lost) upon collision with the inner surfaces of the ionization chamber of the EI source. For many applications, it would be more advantageous to generate an on-axis electron beam, i.e., an electron beam that is coaxial with the resulting ion beam and with the downstream device into which the ions are transmitted such as, for example, a quadrupole mass filter. An axial electron beam may be much more likely to create ions that would have a much higher likelihood of success of being transferred into the downstream device from the EI source.
Therefore, there is a need for ion sources that produce ion beams coaxial with the electron beams that induce ionization, with reduced ion loss.